| Copyright | (C) 2011-2015 Edward Kmett |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Stability | provisional |
| Portability | portable |
| Safe Haskell | Trustworthy |
| Language | Haskell98 |
Data.Functor.Semialternative
Description
Synopsis
- class Functor f => Semialternative f where
- optional :: (Semialternative f, Applicative f) => f a -> f (Maybe a)
- module Data.Functor.Semiapplicative
Documentation
class Functor f => Semialternative f where Source #
Laws:
<!> is associative: (a <!> b) <!> c = a <!> (b <!> c) <$> left-distributes over <!>: f <$> (a <!> b) = (f <$> a) <!> (f <$> b)
If extended to an Semialternativeernative then <!> should equal <|>.
Ideally, an instance of Semialternative also satisfies the "left distributon" law of
MonadPlus with respect to <.>:
<.> right-distributes over <!>: (a <!> b) <.> c = (a <.> c) <!> (b <.> c)
But Maybe, IO, Either aErrorT e mSTM satisfy the alternative
"left catch" law instead:
pure a <!> b = pure a
However, this variation cannot be stated purely in terms of the dependencies of Semialternative.
When and if MonadPlus is successfully refactored, this class should also be refactored to remove these instances.
The right distributive law should extend in the cases where the a Semimonad or Monad is
provided to yield variations of the right distributive law:
(m <!> n) >>- f = (m >>- f) <!> (m >>- f) (m <!> n) >>= f = (m >>= f) <!> (m >>= f)
Minimal complete definition
Methods
(<!>) :: f a -> f a -> f a infixl 3 Source #
<|> without a required empty
some :: Applicative f => f a -> f [a] Source #
many :: Applicative f => f a -> f [a] Source #
Instances
optional :: (Semialternative f, Applicative f) => f a -> f (Maybe a) Source #
One or none.
module Data.Functor.Semiapplicative